The scene where Sean and Will are in his office, and Sean starts talking about his dead wife and her farting antics. These lines were ad-libbed by Robin Williams, which is probably why Matt Damon is laughing so hard. If you watch the scene carefully you can notice the camera shaking, probably due to the cameraman laughing as well.
When Robin Williams won the Oscar for his supporting role, he sent Peer Augustinski, who dubbed his voice in German, a small replica of the Oscar statue with a note saying, "Thank you for making me famous in Germany."
Director Gus Van Sant at one point asked Matt Damon and Ben Affleck to rewrite the script so that Chuckie is killed in a construction accident. Damon and Affleck protested, but reluctantly wrote the scene in. After Van Sant read it, he agreed that it was a terrible idea.
Casey Affleck ad libbed most of his lines. This frustrated Matt Damon, Ben Affleck, and Gus Van Sant during filming - but they later admitted that Casey's improvised lines were much funnier and better than what had been originally written for him.
When Will (Matt Damon) and Sean (Robin Williams) meet for the first time in Sean's office, Will recommends that Sean read Howard Zinn's "People's History of the United States". As a boy, Matt Damon was Zinn's neighbor and provided the voice for the CD recording of that book.
Matt Damon, a former Harvard student, originally intended to make the title character a physics prodigy. He discussed his idea with Sheldon L. Glashow, a Nobel laureate in physics and at the time a Harvard professor. Glashow told him that the premise did not ring true to him and suggested that the main character be a math prodigy instead. He referred Damon to his brother-in-law, Daniel Kleitman, a professor of mathematics at MIT who provided advice on the story. Both Glashow and Kleitman are thanked in the credits.
An earlier draft of the script had Will Hunting being recruited by the government to become a cryptanalyst (based on his mathematical ability). Rob Reiner reportedly reviewed the script and advised Matt Damon and Ben Affleck to eliminate this subplot. However, there is a reference to it in the final script: the scene where Will meets with NSA agents and explains why he doesn't want to work for them.
Initially, producer Harvey Weinstein did not want Minnie Driver at all for the role of Skylar, feeling she wasn't cute enough for the part. Because Gus, Matt and Ben wanted her in the movie, Weinstein ultimately relented and Driver went on to scoop a Best Supporting Actress Oscar nomination for her efforts.
At a WGA seminar in 2003, William Goldman denied the persistent rumor that he was the actual writer of Good Will Hunting: "I would love to say that I wrote it. Here is the truth. In my obit it will say that I wrote it. People don't want to think those two cute guys wrote it. What happened was, they had the script. It was their script. They gave it to Rob [Reiner] to read, and there was a great deal of stuff in the script dealing with the F.B.I. trying to use Matt Damon for spy work because he was so brilliant in math. Rob said, "Get rid of it." They then sent them in to see me for a day - I met with them in New York - and all I said to them was, "Rob's right. Get rid of the F.B.I. stuff. Go with the family, go with Boston, go with all that wonderful stuff." And they did. I think people refuse to admit it because their careers have been so far from writing, and I think it's too bad. I'll tell you who wrote a marvelous script once, Sylvester Stallone. Rocky's a marvelous script. God, read it, it's wonderful. It's just got marvelous stuff. And then he stopped suddenly because it's easier being a movie star and making all that money than going in your pit and writing a script. But I did not write [Good Will Hunting], alas. I would not have written the "It's not your fault" scene. I'm going to assume that 148 percent of the people in this room have seen a therapist. I certainly have, for a long time. Hollywood always has this idea that it's this shrink with only one patient. I mean, that scene with Robin Williams gushing and Matt Damon and they're hugging, "It's not your fault, it's not your fault." I thought, Oh God, Freud is so agonized over this scene. But Hollywood tends to do that with therapists." As of 2009, Matt Damon and Ben Affleck have both co-written one other script each, although not with each other; Damon co-wrote Gerry (2002) with Gus Van Sant and Ben's brother Casey Affleck, and Ben Affleck directed and co-wrote (with his childhood friend Aaron Stockard) the script for Gone Baby Gone (2007). In 2010, Ben Affleck directed The Town (2010), for which he had also co-written the screenplay.
The phone number printed on the sign for the construction company that they are working for is the actual phone number of a Woburn, MA construction company that Matt Damon worked for while going to high school in Cambridge.
The script was originally developed by Castle Rock, the production company of Rob Reiner. When they didn't know what to do with it, filmmaker Kevin Smith took the script to Miramax. It became the highest grossing film in Miramax history until Chicago (2002) topped it.
To date this is the film with the highest US box office gross with Kevin Smith's name attached to it. All his own films that he has written and directed have not grossed more than $35 million at the US box office.
The mathematical equations seen in the opening credits are part of a math technique called "Fourier Analysis" which approximates functions by sines and cosines. It's used a lot in physics and engineering.
In the scene on the park bench, Robin Williams gives an example of love that Will hasn't experienced as "going to hell and back for it." In his next movie, What Dreams May Come (1998), Williams does just that for his love played by Annabella Sciorra after she commits suicide.